The present application relates to bidirectional switches, and particularly to symmetrically-bidirectional power bipolar transistors.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Published US application US 2014-0375287 (which is hereby incorporated by reference in its entirety) discloses (inter al/a) novel bidirectional bipolar transistors known as “B-TRANs.” Further improvements to the B-TRAN device and its modes of operation where disclosed in application Ser. Nos. 14/937,814 and 14/882,316.
One sample embodiment of a B-TRAN can be seen in FIG. 1. Note that the two surfaces of the die are essentially identical.
A sample circuit symbol is shown in FIG. 2. This circuit symbol resembles that of a bipolar junction transistor, except that two base connections are shown. This corresponds to the device structure of FIG. 1, where two different base contact regions are placed on the two surfaces of the die.
FIG. 3 shows one sample embodiment of a B-TRAN drive circuit, as extensively described in the parent applications.
FIG. 4 shows another sample embodiment of a B-TRAN. In this embodiment the trenches contain field plates; the capacitive coupling to the field plates helps to smooth the nearby voltage gradient in the vertical direction.
The preferred modes of operation of the BTRAN are surprisingly complex. To achieve high bipolar gain reliably, in a bidirectional device, the parent applications teach that the following stages of operation can be used.
At turn-on, an initial flow of current is allowed to occur in “diode mode” before bipolar transistor operation begins. In diode mode, the voltage drop across the device is (of course) at least a diode drop; but when base current drive is applied, the forward voltage drop can be reduced to a few hundred millivolts.
At turn-off, base current is disabled first, so that the device is again operating as a diode. After this, the device can be put into the “active off” mode, where one of the two junctions is reverse biased and blocks current.
A further surprising mode taught in the parent application is the “passive-off” mode. A problem with a vully bidirectional device is that the bipolar gain can interfere with current blocking in the off state. To avoid this, the emitter junction on either surface of the device is clamped to avoid any significant forward bias. (Properly, the “emitter junction” referred to here is the junction between either of the (typically n-type) emitter/collector regions and the (typically p-type) substrate.) By keeping the emitter junctions well away from turn-on, minority carrier injection is limited, and the gain of the bipolar transistor does not degrade the breakdown voltage.